Power plant apparatus



J ly 1967 H. E. BURBACH ETAL 3,329,575

POWER PLANT APPARATUS Filed Dec. 27, 1963 United States Patent 3,329,575 POWER PLANT APPARATUS Henry E. Burbach, Avon, and Joseph G. Singer, Bloomfield, Conn., assignors to Combustion Engineering, Inc., Windsor, Conn, a corporation of Delaware Filed Dec. 27, 1963, Ser. No. 333,885 7 Claims. (Cl. 176-65) This invention relates to steam turbine cycles where the steam source is at a low pressure, and in particular to steam turbine cycles using as a power source nuclear reactors.

In vapor generators associated with nuclear reactors steam is generated at relatively low pressures and with very little, if any, superheat. Typical conditions for such a steam generator would be in the order of 600 p.s.i. pressure and 500 F. temperature. In order to simplify and improve the turbine design so that excessive amounts of Wet steam in the low pressure blading can be avoided and that improved efiiciency can be obtained, it has sometimes been found desirable to install separately fired superheaters to increase the temperature of the steam after having left the steam generator.

The design of a separately fired superheater with its steam-cooled furnace is always very difficult because of the high furnace metal temperature encountered. In such a unit heat must be transmitted from the furnace through the gas film on the outside of the tubes, through the tube itself, and through the steam film inside the tube, to the steam proper which is being conveyed therethrough to be heated. The metal temperature of the tube is above that of the steam being heated in a direct relationship with the internal steam film heat transfer coeificient. Inasmuch as this heat transfer coefiicient is poorer for steam than for Water, metal temperatures experienced in furnace tubes are substantially higher in a separately fired superheater than they would be in a water heater or a vapor generator.

Since this is a low pressure system the plant cycle efficiency dictates a low superheater pressure drop. An increased pressure drop across the superheater would mean either raising the pressure of the steam generator, (but in such a plant this steam generator is already at a reasonable design maximum), or lowering the turbine pressure, which would result in a lower turbine efiiciency. The low pressure, and low pressure drop allowed for the separately fired superheater, aggravate the condition of high metal temperature as compared to a superheater designed for higher pressures. The low pressure drop requirement necessitates a reduction in the design mass flow rates through the tube, which in turn, degrades the heat transfer coefficient. In addition, for the same flow rate the heat transfer coefficient is poorer for low pressure steam.

Therefore the design, Where low pressure steam is to be heated in a separately fired superheater which must 'be designed for low pressure drop, is an extremely difficult problem. This requires relatively large furnaces so that the absorption rates through the tubes may be kept to low values, and the extensive use of austenitic steel. The low pressure drop requirement combined the high specific volume occurring with this low pressure steam necessitates the use of large diameter tubes, which inherently require increased wall thicknesses over that required for smaller diameter tubes, designed for the same pressure and temperature. The long headers required to supply all these tubes, together with the low pressure drop, creates a substantial problem in the distribution of steam through the tubes. The maldistri bution of steam therefore encountered is reflected in higher temperature unbalances and therefore a still further increase in the required metal design temperatures. In this respect design conditions are sometimes met which exceed the limits of todays materials, making a workable design impossible.

A separately fired superheater of this type also encounters difiiculties with respect to operating metal temperatures during startup of a system. When steam at low flow rates is passed through the tubes, the low flow results in a very poor heat transfer coefficient and makes it impossible to safely fire the unit without exceeding allowable metal temperatures. Therefore, large turbine bypass systems are installed so that the turbines designed to take superheated steam, can be bypassed until a suflicient flow rate is obtained in the separately fired superheater to permit firing therein, in order to raise the steam temperature above saturation.

A common method of controlling a pressurized water reactor involves maintaining the pressure in the steam generator at such a level that it automatically takes its power requirements from the reactor. In this system the steam generator operates at higher pressure at low loads so that the temperature level of the saturated water is increased, and the heat taken from the reactor is decreased. In such a power plant system operation at low load comprises operating a feed pump to raise the steam generator pressure to a rather high level, and thereafter dissipating this pressure in the turbine throttle valve. Under these conditions, with the vapor generator at a high pressure and the turbine requiring a lower pressure,-

a higher superheater pressure drop could be tolerated. However, the design of the surface which is imposed by the full load condition must be endured.

The high feed pump power requirements for a low load operation are significant detriment to the power plant cycle. Also, the high pressure drop across the throttle valve at low flows imposes a diflicult operating condition on the throttle valve.

It is an object of this invention to provide an im proved power plant, wherein a low pressure steam source will supply a high pressure turbine.

It is a further object to provide a steam turbine cycle wherein an independently fired superheating means may be designed for high pressure steam therein, even though the steam generating source operates at low pressure.

It is a further object to provide a power plant cycle wherein a separately fired superheating means may be satisfactorily operated over a larger load range.

It is a further object to provide a power plant system including a reactor system operating at an average coolant temperature wherein the plant operation at lower load operation is improved.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired, as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawing wherein:

FIGURE 1 is a schematic of a power plant system wherein steam leaving the low pressure vapor generator is compressed before introduction into the separately fired superheater, the compressor being driven by means of a steam tunbine taking extraction steam from the main turbine; and

FIGURE 2 is a schematic showing the instant invention and its application to a supercharged gas turbine-steam turbine cycle.

Referring to FIGURE 1, the primary reactor coolant is circulated through the primary loop 2 by means of circulating pump 4, thereby transferring heat from the re actor core 6 to the steam generator 8. This pressurized water reactor has a negative moderator temperature coefiicient so that it is inherently self-regulating to maintain approximately a constant average fluid temperature therethrough.

Steam is generated in the steam generator 8 with the water being supplied from condensor 10 by feed pump 12 through feedwater heater 14. The steam thus generated at a pressure of about 600 p.s.i., is conveyed to the steam compressor 16 where it is compressed to a pressure of about 1000 p.s.i. This steam at high pressure is then delivered through the separately fired superheater 18 and through the throttle control valve 22 to the steam turbine 20. The steam pressure at the turbine Will be about 850 p.s.i., allowing 150 p.s.i. pressure drop through the separately fired superheater and steam line. The heat input to the separately fired superheater 18 is regulated by firing through burner 17 to obtain the desired steam temperature entering the turbine 20. This steam turbine drives an electric generator (not shown) and the steam leaving the turbine is condensed in condensor 10 from which it is recycled by feed pump 12.

Extraction steam is taken through extraction line 24 and control valve 26 to supply the auxiliary steam turbine 28 which drives the compressor 16. The exhaust from the auxiliary turbine 28 is conveyed through steam line 30 to the condensor 10.

Turbine bypass line 32 containing stop valve 34 is arranged to bypass the steam turbine permitting steam to be conveyed from the vapor generator to the condensor directly without passing through the steam turbine during startup.

The separately fired superheater 18 can now be designed for high pressure steam at 1000 to 850 p.s.i. rather than 575 to 525 p.s.i. which would be the case without the steam compressor. This decreases the specific volume and decreases the pressure drop for a given flow rate through the superheater tubes. Furthermore, since increased pressure drop is available, a higher pressure drop may be realistically taken across the separately fired superheater, thus permitting improved distribution of steam flow among the tubes with consequent temperature unbalance, and higher velocities within the tubes, thereby improving the heat transfer coefficient resulting in lower metal temperatures.

This advantage has a number of ramifications. A smaller furnace with higher absorption rates may be used without exceeding practical metal temperatures and since the furnace is smaller, a decreased quantity of material need be used. Or advantage may be taken of the lower metal temperatures by using lower grade tubing. The improved higher pressure drop design will work to advantage during low load operation, thereby permitting firing the separately fired superheater at lower load resulting in a smaller bypass system. Not only will the costs of this separately fired superheater and bypass system be reduced but a reasonable design may be made for conditions where it was previously impossible to design for low pressure steam. The improved distribution between tubes leads to less uncertainty in design as to the erratic flows through tubes in parallel, therefore leading to a decrease in tube failures and an improved availability.

In this embodiment, the steam turbine is naturally designed for higher pressure in the order of 850 p.s.i. and the compressor is doing maximum work at full load. At this time the steam generator is operating at 600 p.s.i. pressure and the pressure at the compressor outlet is 1000 p.s.i. When reducing load on the system with a fixed control rod position, the pressure in the vapor generator is increased. This increases the saturation temperature of the fluid therein, decreasing the heat taken from the primary fluid being circulated by primary circulating pump 4. This, in turn, feeds back to the reactor 6 having 0. a negative temperature coefiicient, to reduce the power output of the reactor. The required pressure at the turbine inlet is decreased as load is decreased, while the pressure in the steam generator increases; therefore the work which must be done by the steam compressor, drops at a rapid rate. At about percent load and 750 p.s.i. at the turbine inlet, the steam compressor is no longer required. Although the feed pump must increase the pressure in the steam generator to about 850 p.s.i. (about the same that it would to generate the same steam quantity in the system not using a compressor), this pressure is not dissipated through a throttle valve as it would be in a conventional system, but is here utilized in the turbine. In a conventional system, the turbine would be designed for about 500 p.s.i. at full load and would require a turbine inlet pressure of about 375 p.s.i. at 75 percent load.

Conversely the instant invention may be applied to peaking operation. In such an application the nuclear reactor and vapor generator combination would be designed for a full load output at a given operating pressure in the vapor generator without the steam compressor 16 operating. In order to increase the output of the plant the steam compressor would be operated, the effect of this being to reduce the pressure in the vapor generator and to increase the pressure at the turbine inlet. The decreased pressure in the vapor generator will act through the primary fluid circulating loop to increase the output of the reactor, and therefore the evaporation of the vapor generator would be increased. Since the operating pressure of the pressure parts in the separately fired superheater 18 and the turbine 20 will be increased during peaking operation, this must be considered in the initial design. Although the overloading of the vapor generator will increase steam velocities and possibly carryover, thereby leading to some solids deposition in the superheater 18. This could be tolerated since peaking operation is by nature short term operation.

Inasmuch as it is difficult to predict the operation of nuclear reactors with accuracy, particularly as to temperature gradient within the same, it is often found that in actual operation the reactor is capable of delivering greater than design output. Since this power must be removed at a higher operating temperature level due to the increased pressure in the vapor generator, the control rods must be withdrawn to achieve the output. This, however, reduces the tolerance in the reactor design for fuel burnup, thereby decreasing the life at which the reactor may operate at full power. The aforementioned method of operating a reactor for peaking operation can be applied in this case to increase the full power life of the system.

Referring now to FIGURE 2 the steam cycle is similar to that of FIGURE 1 in that water is pumped from condenser 10 through feedwater heater 14 into the vapor generator 8 by feed pump 12. Compressor 16 increases the pressure of the steam leaving the vapor generator 8 after which the temperature of the steam is increased in the separately fired superheater 36. This steam is then conveyed through turbine throttle valve 22 and turbine 20 during normal operation, or through bypass valve 34 and bypass line 32 during startup operation.

The steam compressor 16 is driven by a gas turbine combination where forced draft fan 38 introduces air into the air compressor 40, which supercharges the air compressor. The compressed air is conveyed to the combuster 42 wherein fuel 44 is burned increasing the temperature of the gases which are then conveyed to the gas turbine 46. The gas leaving the gas turbine will be at about 850 F. and contain in the order of 17 percent oxygen, since high excess air is required in the operation of gas turbines in order to avoid excessively high temperatures. This gas is then conveyed to a separately fired superheater 36 where it is used as combustion supporting air for fuel 48 which is burned therein, Exhaust gases from the separately fired superheater are conveyed through duct 50 to stack gas cooler 52 wherein heat is transferred to the feedwater entering the vapor generator 8.

During low load operation, the gas turbine is not re quired and dampers 54 are opened permitting the forced draft fan 38 to supply air directly to the separately fired superheater.

While we have illustrated and described a preferred embodiment of our invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. We therefore do not wish to be limited to the precise details set forth but desire to avail ourselves of such changes as fall within the purview of our invention.

What we claim is:

1. A power plant apparatus comprising: a vapor generator operative to supply steam only at low pressure; a pressurized water nuclear reactor as a heat source for said vapor generator with the pressurized water flowing as the heating medium in heat exchange relation through said vapor generator; a feedwater pump connected to supply water to said vapor generator at low pressure; a steam compressor receiving steam from the vapor generator and operative to increase the pressure of the steam passing therethrough; a separately fired superheater connected to receive the high pressure steam only from the compressor; said separately fired superheater being fired with fossiLfuel, and the pressure of the steam where heat for superheating is being absorbed being higher than the pressure of the water where heat for evaporation is being absorbed and a steam turbine connected to receive high pressure high temperature steam from the separately fired superheater.

2. An apparatus as in claim 1 including also: variable throttling means operative to restrict stream flow from the separately fired superheater to the turbine; an auxiliary steam turbine connected to drive the compressor; and at least one steam extraction point from the steam turbine connected to supply steam for driving the auxiliary turbine.

3. A power plant system comprising: a nuclear reactor having a negative temperature coefiicient; a primary coolant loop; a vapor generator operatively connected to the reactor by means of the primary coolant loop, having the characteristic that increased operating pressure in the vapor generator operates through the primary coolant to decrease the power output of the nuclear reactor; a compressor receiving steam from said vapor generator; a fossil-fuel fired heat exchanger operative to increase the temperature of steam passing therethrough connected to receive steam from the compressor; a steam turbine connected to receive high pressure high temperature steam from the heat exchange means; and throttling means connected to restrict the flow of steam from the vapor generator through the compressor and heat exchange means to the turbine.

4. A power plant apparatus comprising: a vapor generator capable of generating steam only at low pressure; a nuclear reactor operatively connected to said vapor generator as the heat source therefor; a compressor connected to receive steam from the vapor generator at low pressure; a separately fired superheater connected to receive steam at high pressure [from the compressor; a steam turbine connected to receive steam at high pressure and high temperature from the separately fired superheater; throttling means interposed between the vapor generator and the turbine to restrict steam flow passing from the vapor generator to the turbine; a gas turbine comprising an air compressor, a combuster and a gas turbine, having exhaust gases at high temperature and with high oxygen content; means for conveying the gas turbine exhaust gases to the separately fired superheater as combustion supporting air; and means for firing fuel for combustion in the separately fired superheate-r the combustion taking place being effective to transfer heat to the steam passing therethrough.

5. An apparatus as in claim 4 including also: means for supplying combustion supporting air to the separately fired superheater independent of the gas turbine.

6. An apparatus as in claim 5 including also: a stack gas cooler; means for passing water therethrough to be heated; means for conveying the heated water to the vapor generator; means for conveying combustion prod ucts from the separately fired superheater to the stack gas cooler wherein they are passed in heat exchange relationship with the water passing therethrough.

7. In a power plant system having a nuclear reactor having a negative coefiicient, a vapor generator, a primary fluid circuit conveying heat from the reactor to the vapor generator, the vapor generator having the characteristic that changing the operating pressure of the vapor generator feeds back through the primary coolant circuit to change the reactor output in inverse relation to the operating pressure of the vapor generator, a fossil-fuel fired superheater receiving steam from said vapor generator; a steam turbine receiving steam at low pressure from the fossil-fuel fired superheater, the method of operating comprising: circulating primary coolant to transfer heat from the reactor to the vapor generator, generating steam therein at low first pressure, conveying the steam through said fossil-fuel fired superheater to the steam turbine at a second pressure to generate power thereafter; increasing the steam output of the reactor and the power output of the plant by continuing circulation of primary coolant to transfer heat from the reactor to the vapor generator, generating steam at a pressure lower than said first pressure, compressing this steam to a high pressure higher than said second pressure, and conveying the steam at said high pressure to a turbine for the generation of power.

References Cited UNITED STATES PATENTS 2,939,286 6/1960 Pavlecka.

2,970,434 2/ 1961 Warren 60-104 2,997,032 8/1961 Wedel 122-33 3,210,943 10/1965 Acklin 60-104 3,231,475 1/1966 Kagi 60-104 X RUEBEN EPSTEIN, Primary Examiner. 

1. A POWER PLANT APPARATUS COMPRISING: A VAPOR GENERATOR OPERATIVE TO SUPPLY STEAM ONLY AT LOW PRESSURE; A PRESSURIZED WATER NUCLEAR REACTOR AS A HEAT SOURCE FOR SAID VAPOR GENERATOR WITH THE PRESSURIZED WATER FLOWING AS THE HEATING MEDIUM IN HEAT EXCHANGE RELATION THROUGH SAID VAPOR GENERATOR; A FEEDWATER PUMP CONNECTED TO SUPPLY WATER TO SAID VAPOR GENERATOR AT LOW PRESSURE; A STEAM COMPRESSOR RECEIVING STEAM FROM THE VAPOR GENERATOR AND OPERATIVE TO INCREASE THE PRESSURE OF THE STEAM PASSING THERETHROUGH; A SEPARATELY FIRED SUPERHEATER CONNECTED TO RECEIVE THE HIGH PRESSURE STEAM ONLY FROM THE COMPRESSOR; SAID SEPARATELY FIRED SUPERHEATER BEING FIRED WITH FOSSIL-FUEL, AND THE PRESSURE OF THE STEAM WHERE HEAT FOR SUPERHEATING IS BEING ABSORBED BEING HIGHER THAN THE PRESSURE OF THE WATER WHERE HEAT FOR EVAPORATION IS BEING ABSORBED AND A STEAM TURBINE CONNECTED TO RECEIVE HIGH PRESSURE HIGH TEMPERATURE STEAM FROM THE SEPARATELY FIRED SUPERHEATER. 